Question 249941
working alone, it takes the scientists 4 hours to create one dose of formula. 
the mad scientist's slightly mad assistant Igor can make one dose in 12 hours. 
how long, working together, will it take the mad scientist and Igor to create 
one dose of formula?
-----------------
The scientist makes 1/4 dose per hour
The assistant makes 1/12 dose per hour
Together, they make 1/4 + 1/12 per hour
= 3/12 + 1/12 = 1/3 dose per hour
--> 3 hours per dose.
----------------------
Use the shortcut, product/sum
4*12/(4+12) = 48/16 = 3